This invention relates to an apparatus for transferring disks such as semiconductor wafers between a cassette of a conventional type and a pallet for holding them vertically with both their surfaces exposed for processing. More particularly, this invention relates to such an apparatus for disks with a small central hole.
Semiconductor wafers are fragile and are easily rendered useless by contamination or abrasion. For this reason, they are usually carried in a cassette when they are transported from one work station to another for processing. A cassette used for such a purpose is shaped generally like a box or a cage having walls with grooves at a uniform pitch. A plurality of (typically 25) disks are carried in one of such cassettes, maintained vertically and parallel to one another with their central holes aligned. Cassettes of this type have been described, for example, in U.S. Pat. Nos. 4,311,427, 4,471,716, 4,493,418 and 4,743,159.
Although the disks may be kept in such a cassette for some kinds of processing, they are often required to be transferred onto what is frequently referred to as a pallet, which is a planar disk holder having a plurality of circular holes with grooves such that the disks can be supported vertically thereat with both surfaces substantially entirely exposed. Such a disk holding device has been described, for example, in U.S Pat. Nos. 4,595,481, 4,634,512 and 4,880,518, and a disk gripper for gripping disks by the peripheries of their central holes for robotically transferring them sequentially between a cassette and a pallet has been described, for example, in U.S. Pat. No. 4,957,406.
Modern progress in technology has made it possible to reduce the size of the central hole of wafers. Nowadays, semiconductor wafers with central holes with diameter as small as about 8 mm are not uncommon. With central holes of the disks thus reduced in size, disk grippers of the type having a rod with devices thereon for gripping the inner peripheries of the disks are not practical because such devices for securely grabbing the disks cannot be made very small.